1. Field of the Invention
The present invention relates to a leather-like sheet capable of providing balls having a sufficient surface abrasion resistance and an excellent non-slip properties because of its high sweat-absorbing ability, and more particularly to a leather-like sheet for balls which is suitable as materials for various balls such as basketball, American football, handball and rugby ball.
2. Description of the Prior Art
Hitherto, a number of leather-like sheets for ball materials have been proposed as a substitute for natural leathers. Materials for balls are frequently required to have good non-slip properties. As the production method of a non-slipping leather-like sheet, there has been proposed, for example, a method of coating a surface of a base fabric with a coating composition containing a polyurethane resin having hydroxyl groups in its molecule, a liquid rubber having hydroxyl groups in its molecule, an inorganic or organic filler, and an isocyanate prepolymer (Japanese Patent Publication No. 7-30285, pages 2-3). However, the leather-like sheet produced by the above method is not suitable for long-term continuous use, because the ability to absorb sweat is easily lost when exceeding the absorption limit, although the coated surface absorbs sweat from hands to some extent. Also, since the resin, etc. having absorbed sweat becomes plasticized and the original hand feel is lost, the proposed leather-like sheet is not suitable for continuous use.
In another proposed method, a nonwoven fabric impregnated with a coagulated elastomeric resin is sliced along its intermediate layer (Japanese Utility Model Application Laid-Open No. 63-197475, pages 2-3). However, the leather-like sheet produced by the above method is very tacky and soft to exhibit a low surface strength, thereby failing to withstand the use under severe conditions, particularly, as in basketball game.
In still another proposed method, the surface of a heat-foamed product made from a rubber material incorporated with gelatin is made into a porous structure by removing a part of its surface skin layer and then removing gelatin by hot water from the surface (Japanese Patent Application Laid-Open No. 63-152483, pages 2-3). However, the leather-like sheet having a porous structure made by removing gelatin using hot water or having a porous surface layer containing a penetrant is also very tacky to have an insufficient abrasion resistance and not suitable as materials for balls.
There has been also proposed a leather-like sheet for balls which comprises a porous base layer made of a fiber-entangled fabric, a porous elastomer and a penetrant and a porous surface layer formed thereon. Microholes are formed on the porous surface layer and the penetrant is present in microholes (Japanese Patent Application Laid-Open No. 2000-328465, pages 2-3). The proposed leather-like sheet for balls is excellent in the initial sweat-absorbing ability because microholes are formed throughout its entire surface. However, in surface-pebbled balls such as basketball made thereof, the sweat-absorbing ability tends to be reduced during long-term use to make the handling properties and surface touch insufficient, because dirt easily deposits between pebbles and deposited dirt is difficult to be removed. Since microholes are present over the entire pebbled surface, the pebbled surface is likely to lose its original patterns during long-term use if the number of the microholes is large. If the number of the microholes decreases to avoid the lost of the original surface pebbled pattern, the sweat-absorbing ability is reduced, and additionally, the handling properties and surface touch tend to become insufficient.
There has been also proposed a sweat-absorbing game ball that comprises a polyurethane-impregnated fibrous material and an outer coating of wet-coagulated polyurethane thereon. The surface of the outer coating includes a plurality of projecting pebbles and valleys therebetween. A plurality of openings are formed on the side surfaces of the pebbles (U.S. Pat. No. 6,024,661, pages 4-5). However, since only the side surfaces of the pebbles are made porous by a plurality of openings, dirt easily deposits to valley between the pebbles, particularly to the side surface of the pebbles and is difficult to be removed. During long-term use, dirt accumulates to reduce the effect of sweat absorbing and make the surface touch poor.